The process is called probing. It has the objective of measuring your periodontal pockets in order do determine your gengival health. Bigger values mean that that specific spot is not in good conditions from a periodontal health standpoint due to dental plaque acummulation and periodontal inflamation. The reference values are as follows : below 2/3mm its considered healthy above that its considered that that site is having an active inflamation process happening due to bad dental plaque control ( sometines deeper probing values do not reflect active inflamation due to dental plaque but from other factors such as occlusal trauma), the bigger the value the worst ir is.
Pratical example : 3 4 5 means that the gengival pocket at site of the tooth that is closest to its adjacent "backtooth" is 3mm , the pocket in the middle is 4 mm deep and pocket closest to the front adjacent tooth is 5 mm.
HERE YOU HAVE THE MEANING OF THE NUMBERS.