"personal guarantee"

It means that an individual is personally liable for the debt (or another company if its a company or cross guarentee) over and above the specific security the loan is secured on (if any)

Depends firstly on whos giving the personal guarentee, their assets and is the PG specifically backed by any assets.

So a PG that identifies specific assets over which lenders have a claim if the PG is called on obviously adds to the strength of the security. A particular example was the FF loan on AC where guarentee was backed by a share portfolio which enabled recovery of the loan.

Otherwise, its a question of whether the PG can be pursued and that seems to be very hit & miss. If the PG is from a HNW individual then in theory there should be something to pursue, though how easy this is depends on how cooperative they are and whether they are willing to expend resources to avoid/limit their obligations. In many cases PG turn out to be worthless as the guarantor doesnt have the resources to make any meaningful contribution, has multiple creditors so any return is diluted, 'avoids' the claim, goes bankrupt etc. Again depends on the attitude of the borrower. I have one on FK who has been making small contributions to his debt from a low salary for a while now because he believes that is right thing to do, others dont take that attitude.

Overall a PG probably adds little to the security. It does mean that a loan to a company has an additional recovery route  and ties a director/owner to the loan but one that generally turns out to be poor or useless in above average number of cases.