Deferred tax

Dear Sir,

Please help me understand this statement.
“Deferred tax should be recognised on the revaluation of PPE even if any tax due on the gain made on sale of any asset can be deferred by being ‘rolled over’ against the cost of a replacement asset.”

I didn’t understand the roll over part. Does it mean some amount is reduced from cost of investment which would reduce tax relief? (which in turn would imply tax paid on sale proceeds).

Please help me understand this.

Thanking you in advance.

Regards,
Rajvir Singh Oberai.