Without going too deep into mixer topologies, the channel fader sets the gain (you might also think of it as the level or volume, though it’s not quite the same thing) of the sound going into the mix bus (also called the 2-bus or the master channel). Placing effects before the fader (pre-fader) mean that those effects will “hear” the same level, no matter what the fader is set to. Placing effects after the fader (post-fader) will mean that thsoe effects will “hear” a level depending on what the fader is set to. This is particularly noticeable with effects such as compression, which respond differently depending on the level of the sound. If you set up yoru compressor pre-fader, then it will behave the same no matter what the fader is set to. On the other hand, if you set up your comrpessor post-fader, then higher fader gain will result in more compression and lower fader gain will result in less comprssion. In effect, you will use the fader to simultaneously set the audible volume of the sound in the mix AND “drive” the compression. Normally this is not such a good idea beacuse it makes it more difficult to fine-tune the mix (changing the volume changes the compression too).
Post-fader effects are typically not used often, except for sends (also called “aux sends” or “FX sends”). The “send” effectively duplicates the sound and sends one copy to the send channel (the other copy is sent through the original channel as normal). If a wet reverb is applied to the send channel, you’ll have two channels making sound – the original “dry” (no reverb) channel, and the “wet” (reverb) send channel. If the send is post-fader, then the sound level that is sent to the reverb depends on the fader setting. This way, if you adjust the fader (to fine tune the mix, or perhaps automate a fade in or out) the RELATIVE level of the reverb stays the same. On the other hand, if the send it pre-fader, the absolute level of the reverb stays the same (so if you turn the fader all the way down, you’ll still hear some reverb, and if you turn the fader all the way up, you’ll hear less reverb relative to the original sound).