A pip is the standard unit for measuring Forex price moves. For most pairs it is the fourth decimal place. Many platforms show a fifth decimal too: a tenth of a pip.
Pairs quoted in Japanese yen have fewer decimals, so a pip is the second decimal place instead of the fourth.
Forex position size is measured in lots. Most platforms also let you trade fractions of a lot, so you can size positions precisely.
Pip value is how much money each pip is worth for your position size. For pairs quoted in US dollars, it scales directly with the lot size.
Professional traders choose the risk first, then calculate the size. Never the other way round.
Pips measure the move, lots set the size, and together they decide what you gain or lose.
