A flat symbol indicates that the pitch should be lowered by a half step.
The flat symbol is placed before the note on the staff to impact that note; when we write the letter name using words, the flat goes after the letter. When writing words, a flat is indicated using the lower case “b”, such as Bb.
The easiest way to understand the impact to pitch is to think of the piano keyboard. A flat moves the pitch to the key to the immediate left. Here are some examples.
G (white key) becomes Gb by moving one key to the left (black key).
Here is what G and G flat look like as notes on the staff.
F (white key) becomes Fb also by moving one key to the left. In this case a white key moves to another white key, but it is still a flat (half step lower) because it is the key to the immediate left of the original note (F flat happens to be enharmonic to E).
Here is what F and F flat look like as notes on the staff.