Hey, Savannah! This isn't exactly an Organic Chemistry question, but we'll give you a pass this time :P
Remember that the trp operon is a collection of genes that are responsible for creating the amino acid tryptophan. However, bacteria don’t always need to produce their own tryptophan - they can also get it from the environment they’re in. So, if the tryptophan concentration in the bacterium is high (so lots of tryptophan) it no longer needs to make more tryptophan using the operon. In this case the bacteria uses a trp repressor to turn off the trp operon so that it wont produce more tryptophan when there is already enough of it in the environment. The best part about this system is that the repressor can be turned on and off in response to the amount of tryptophan present in the environment. So, when there is a lot of tryptophan, the tryptophan will bind to the repressor (which turns the repressor on), then the repressor can bind to the operon and shut off gene expression. But, when there is little to no tryptophan, there is not enough tryptophan left in the bacterium to bind to the repressor (turning the repressor off). When the repressor is turned off, it wont bind to the operon - and then the genes will be expressed and they will promote the production of more tryptophan.