Complex sentence: When a creature's Monstrous ability is activated, it gains a specified number of +1/+1 counters and sometimes has an additional effect on the game state, such as destroying lands or other creatures.
Split into simpler sentences: When a creature's Monstrous ability is activated, it gains a specified number of +1/+1 counters. Additionally, some Monstrous creatures also have triggered, one-time effects on the game state, such as destroying lands or other creatures.