Okay, imagine you have a big box where you keep all your toys. Now, imagine that some of your toys are related to each other in some way. For example, all your cars and trucks might be related because they are vehicles. Another example could be all your animal toys are related because they are all animals.
Now, imagine that the big box is your brain and all your thoughts are your toys. Loci communes are like little groups of toys (or thoughts) that are related to each other.
So, instead of just having a random bunch of thoughts in your brain, you have these groups of thoughts that are similar to each other. When you think of one thought in a group, it can trigger other related thoughts in that group.
Just like how you might pick out all your car and truck toys to play with together, your brain can pick out these groups of related thoughts to use together when you're thinking or making decisions.