I believe that this line means something along the lines of "talk is cheap." The line is spoken in Chapter 18.
In that chapter, Jaggers, the lawyer, has come to buy out Pip's apprenticeship. He is doing this on the behalf of Magwitch, but Jaggers can't say that.
When Jaggers first brings it up, Joe says that he would not take any money. He says
"Lord forbid that I should want anything for not standing in Pip's way,"
Jaggers asks him again and Joe says again he doesn't want any money. Then Jaggers says "your" line.
I think what it means is that it's alright to talk big (brag) but it is better to act on what you say and keep your word (holdfast).
Does that make sense to you?
In Ch.18 Jaggers under Magwitch's instructions has come announce Pip's "great expectations." Pip has been apprenticed to Joe for the past four years, if Pip had to be groomed as a "gentleman" he would first have be relieved from his apprenticeship to Joe. So, Jaggers asks Joe whether he needs some money to release Pip from the contract of apprenticeship. To Jaggers' surprise, Joe very generously and magnanimously replies in the negative:
"Now, Joseph Gargery, I am the bearer of an offer to relieve you of this young fellow your apprentice. You would not object to cancel his indentures, at his request and for his good? You would want nothing for so doing?"
"Lord forbid that I should want anything for not standing in Pip's way," said Joe, staring.
"Lord forbidding is pious, but not to the purpose," returned Mr Jaggers. "The question is, Would you want anything? Do you want anything?"
"The answer is," returned Joe, sternly, "No."
Jaggers mistakes Joe's generosity for pride and thinks that Joe is boastfully rejecting any offer of money from him. It is then that Jaggers reminds him to be prudent and accept whatever money is offered to him. The maxim, "Brag is a good dog, but Holdfast is a better," means that it is always better to be prudent and exploit a situation for one's own financial advantage than to let it slip by.