Girls, 38 weeks pregnant... Explain to me already humanely what contra…

Girls, 38 weeks pregnant... Explain to me already humanely what contractions are and how to understand them. Sometimes the belly strains itself, pulls at the lower abdomen like during menstruation, but it hurts and lets go, without any intervals. I'm about to give birth, but I still don't understand what I should feel and when I really need to go to the maternity ward. I'm afraid of just missing the moment and realizing that I'm giving birth only when it will be too late. And how do you count these contractions? Downloaded an app, but I don't even know when to press and what to count there.