Ordinary cells copy themselves and function as part of a heart or brain or whatever. Sometimes cells mutate (which means there are slight errors / alterations) in the copies that are made. The older you get the more your cells mutate (aging is a result of mutating cells). Cancerous cells are one kind of mutation where the cells don't function as part of a heart of brain or whatever it was they were supposed to function as. Instead they just take up room (so there is less room for healthy cells) and they replicate at a higher rate producing more cancerous cells.

A tumor is a cluster of cancerous cells. Tumors can be benign (where the cells are clustered together) or malignant (where the cancerous cells are breaking off and floating round the body looking for somewhere new to colonise).

Best case: There is a benign tumor and the tumor is located somewhere where it is possible to operate to remove it all. In that case the surgeons can go straight in and attempt to remove it. Sometimes that isn't possible though because of the location of the tumor. Sometimes the surgeons can't get to it because they would do too much damage to important structures around the tumor. Sometimes the surgeons can't remove it because it is pushing up against important structures around the tumor that would be damaged if they removed it.

In those later cases chemo can help shrink the tumor so that it is possible to operate to remove it.

When the cancer is malignant then chemo is given to help prevent the spreading of the cancerous cells. The idea is to try and kill them before they set up shop somewhere else in the body. They will also attempt to operate to remove the tumor if possible.

So whether chemo is given before or after depends... Sometimes with the operation they are only able to remove part of it so they will give chemo afterwards to help kill off the bits they couldn't remove too...