This is the 658th most frequent Finnish word.
monissa
"Monissa" means "in many" in English.
Here, 'monissa' is used as a plural locative form to indicate 'in many'. It modifies 'kaupungeissa' ('in cities'), expressing the concept of abundance.
Monissa kaupungeissa on kauniita puistoja.
In many cities, there are beautiful parks.
In this sentence, 'monissa' indicates participation in a variety of projects (plural context), denoting 'in many of'. It refers to the plural locative form of the word 'moni' as it modifies 'projekteissa' ('in projects').
Hän on ollut mukana monissa projekteissa.
He/She has been involved in many projects.
'Monissa' here is used to describe a wide range of scenarios as 'in many', specifically modifying 'tilanteissa' ('in situations').