I've read a conjecture that it is a reference to Godric's Hollow, or that it is the grounds on which a battle is fought, or that it means something that happens on All Hallows Day- the day after Halloween.
I have also read that "Hallow" can refer to a religious icon, such as the Holy Grail or a piece of the cross.
So my latest theory is that Hallows means the Horcruxes.
Edited to add:
What does 'Deathly Hallows' mean?
Any clarification of the meaning of 'Hallows' would give away too much of the story - well, it would, wouldn't it? Being the title and all. So I'm afraid I'm not answering.
This is from the JKR official site. If "Hallows" is as simple as the Horcruxes, then explaining the meaning would not give away the story- right? After all, we already know that searching for the Horcruxes is involved in the story.