What does this mean?

[coppicecru]

Viewed a property to buy but has the below covenant which estate agent cannot explain to us, any ideas?
COVENANT by the Purchaser for himself and the persons deriving title
under him not to do anything on the property thereby conveyed which
might affect the stability of the adjoining property or bridge of the
Vendor Company.

Not sure what 'bridge of the Vending company means' , depends who built the house in the first place - if it was ages and ages ago it may be unenforceable anyway. The rest is just not to cause damage to the neighbouring structure should you extend or modify. Which you wouldn't do anyway or if you did you would need to make good as per the Party Wall Act.
Your solicitor can explain it.

Your conveyancer will advise

My first thought is that the house next door was once or is owned by the bloke who originally owned your block, that next door is higher than yours and they don’t want you to dig a whacking great hole on your fenceline that might disturb their foundations.

Or there is a creek you can’t dam to cause flooding and erosion.

But, I know nothing.

There's clearly a bridge nearby. You can't do anything that might affect the property that joins onto this one (presumably its semi detached or terraced) or the bridge.