Not sure if you know exactly what a bid is, so let me try to explain it:
You're going through rush and at the same time that the different houses are narrowing down their list of PNM's, you're narrowing down the houses. Finally you'll have it narrowed down to two houses, and you go to a party with each of the two on one day, which is called preference night. Then, on your preference card (or whatever you use), you "pref" these houses, ranking the one you like the best first and the next one second. Some girls choose to intentionally single pref, which means that they only put their favorite on the card and nothing else. At my school, there are always a handful of girls who don't get a bid because they intentionally single preffed and that house cut them (at my school, the system is designed that if the house you preffed cuts you, you automatically go to your second choice).
So the next day (or the day after or whenever according to your rush schedule), you will receive what is called a bid from one house. It will either be your first or second choice house from preference night. For bid day, you show up at that house that you received the bid from and you'll get to do fun stuff with your new sisters and the rest of the girls receiving bids from that house. You'll get a cute bid-day T-shirt and you'll probably get a present or two. But the purpose of bid day is that you receive your bid card from the sorority. What a bid card is is basically an invitation to pledge (join) the sorority, and you can either accept by signing it, or decline by not signing. Most girls are happy with the house that they wound up with at the end of rush and choose to sign the bid card. Once you sign, you are officially a new member of that organization. It's such a fun and exciting day for everyone!
Well I hope this little dissertation helped put some things into perspective for you

. If you have any questions, don't hesitate to ask. Also, remember that during recruitment, you'll have Recruitment Counselors (Rho Chis) who will be there to assisit you in whatever you need to ensure that everything goes smoothly.