Did the Old Testament prophets understand the prophecies they wrote about Christ?
No, the Old Testament prophets did not fully understand the prophecies they wrote about Christ. They searched diligently, inquiring what manner of time the Spirit of Christ in them signified, but God did not reveal the full meaning; Daniel was told to shut up the book until the end times.
Les explains from I Peter 1:10-12 that the Old Testament prophets, who prophesied of the grace that should come, inquired and searched diligently. They were searching what manner of time the Spirit of Christ in them was signifying when He testified beforehand of the sufferings of Christ and the glory that should follow. They couldn't put it all together. Les uses Daniel as an example: Daniel asked the Lord what it all meant, and the Lord told him to shut up the book, for it was not for him to know, but at the end of the time they will know. Les suggests the glory spoken of refers to the glory of Christ's Kingdom that would come, though we also understand from the New Testament that He was glorified when He defeated Satan at the cross. So the prophets wrote by inspiration but did not fully understand the timing and fulfillment of their prophecies.
Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow.