Seriously, how amazing is the female body? When the uterus starts preparing for childbirth, the large muscle contracts, causing the belly to harden for a moment before softening. When you have a contraction, the belly becomes hard and protrudes. The contraction starts from different parts of the uterus. They spread irregularly across it and can feel uncomfortable, but usually don't hurt. Some may still find it comforting to place, for example, a heating pad for the contractions.

Contractions usually come irregularly once or a few times an hour. They often disappear when you rest or relax, but some people feel contractions even during rest.
How many contractions one has and how they are perceived varies from person to person. Often, contractions are felt from the middle of pregnancy.

Source: https://www.1177.se/barn–gravid/graviditet/graviditetsbesvar-och-sjukdomar/sammandragningar-och-varkar/