This is the 182nd most frequent Spanish word.
próximo
Next; upcoming.
In this sentence, 'próximo' acts as an adjective, specifying the immediate next Monday as the time for the event.
El próximo lunes tenemos una reunión importante.
Next Monday we have an important meeting.
In this context, 'próximo' is used to describe the bus that follows directly in the sequence of arriving buses.
El autobús que tomaremos es el próximo en llegar.
The bus we will take is the next one to arrive.
Here, 'próximo' describes the immediate succeeding chapter in a sequence, emphasizing its proximity in order.