Streak is a number of consecutive days you upload your photos to tookapic. Keeping the Streak up is not necessary to complete your 365 project. But from what we observe, users that do upload every day or every other day have much bigger chance to actually complete the challenge.
Best practice is to upload photos on the same day you take them. That way you'll be sure your Streak number grows. The ideal scenario is to have streak of 365 on 365th day of your project.