This article is meant to try to give you a mental image of how texture units are setup in WebGL. There is a similar article on attributes.

As a prerequisite you probably want to read How WebGL Works and WebGL Shaders and GLSL as well as WebGL Textures

Texture Units

In WebGL there are textures. Textures are 2D arrays of data you can pass to a shader. In the shader you declare a uniform sampler like this

uniform sampler2D someTexture;

But how does the shader know which texture to use for someTexture?

That's where texture units come in. Texture units are a global array of references to textures. You can imagine if WebGL was written in JavaScript it would have some global state that looks like this

const gl = {
  activeTextureUnit: 0,
  textureUnits: [
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
    { TEXTURE_2D: null, TEXTURE_CUBE_MAP: null, TEXTURE_3D: null, TEXTURE_2D_ARRAY: null, },
  ];
}

You can see above textureUnits is an array. You assign a texture to one of the bind points in that array of texture units. Let's assign ourTexture to texture unit 5.

// at init time
const ourTexture = gl.createTexture();
// insert code it init texture here.

...

// at render time
const indexOfTextureUnit = 5;
gl.activeTexture(gl.TEXTURE0 + indexOfTextureUnit);
gl.bindTexture(gl.TEXTURE_2D, ourTexture);

You then tell the shader which texture unit you bound the texture to by calling

gl.uniform1i(someTextureUniformLocation, indexOfTextureUnit);

If activeTexture and bindTexture WebGL functions were implemented in JavaScript they'd look something like:

// PSEUDO CODE!!!
gl.activeTexture = function(unit) {
  gl.activeTextureUnit = unit - gl.TEXTURE0;  // convert to 0 based index
};

gl.bindTexture = function(target, texture) {
  const textureUnit = gl.textureUnits[gl.activeTextureUnit];
  textureUnit[target] = texture;
}:

You can even imagine how other texture functions work. They all take a target like gl.texImage2D(target, ...) or gl.texParameteri(target). Those would be implemented something like

// PSEUDO CODE!!!
gl.texImage2D = function(target, level, internalFormat, width, height, border, format, type, data) {
  const textureUnit = gl.textureUnits[gl.activeTextureUnit];
  const texture = textureUnit[target];
  texture.mips[level] = convertDataToInternalFormat(internalFormat, width, height, format, type, data);
}

gl.texParameteri = function(target, pname, value) {
  const textureUnit = gl.textureUnits[gl.activeTextureUnit];
  const texture = textureUnit[target];
  texture[pname] = value; 
}

It should be clear from the example pseudo code above gl.activeTexture sets an internal global variable inside WebGL to an index of the array of texture units. From that point on, all the other texture functions take a target, the first argument in every texture function, that references the bind point of the current texture unit.

Maximum Texture Units

WebGL requires an implementation to support at least 32 texture units. You can query how many are supported with

const maxTextureUnits = gl.getParameter(gl.MAX_COMBINED_TEXTURE_IMAGE_UNITS);

Note that vertex shaders and fragment shaders might have different limits on how many units each can use. You can query the limits for each with

const maxVertexShaderTextureUnits = gl.getParameter(gl.MAX_VERTEX_TEXTURE_IMAGE_UNITS);
const maxFragmentShaderTextureUnits = gl.getParameter(gl.MAX_TEXTURE_IMAGE_UNITS);

Each are required to support at least 16 texture units.

Let's say

maxTextureUnits = 32
maxVertexShaderTextureUnits = 16
maxFragmentShaderTextureUnits = 32

This means if you use for example 2 texture units in your vertex shader there are only 30 left to use in your fragment shader since the combined maximum is 32.