1.
mountains, many mountains
Multiple mountains; a range of mountains.
2.
greatly, very much (wanting to)
Used to express strong desire, often followed by 'but...' to indicate constraint.
山々 has two distinct meanings formed by reduplication of 山.
Meaning 1 - mountains
Literal plural/collective meaning "mountains" or "many mountains."
Meaning 2 - very much
Adverbial expression meaning "very much wanting to" but implies a constraint.
Usage
The adverbial sense always implies that despite strong desire, something prevents the action. It's often used politely to decline or explain why something can't be done.