Any solution having lower osmotic pressure as compared to another solution is called as a hypotonic solution and the reason that a solution is hypotonic is that it has more water and lesser dissolved solute in it.
If a cell is placed in a hypotonic solution, the amount of water content in the cell would be lesser as compared to that of the solution and hence, the water will enter the cell due to which the cell will swell and may even burst.
The water molecules can enter the cell through the semipermeable membrane and in case of plant cells, the presence of cell wall may prevent it bursting, however, the animal cells lack the cell wall and hence if kept in hypotonic solution for long these cells may burst.
