Your site would probably be down if you were over the space limit/bandwidth etc and more things
Things that might use up your space would be if you are using digital photo images and some people don't resize them/optimize them for webpages and they end up being 1 to perhaps 3mb's
Just be careful what you use on your site. If site is just an easy site and want to keep it simple if you have the free subdomain or such then great
If you want to expand can look elsewhere
5mb is fine if want just a small little easy site but if for business and more elaborate would purchase a domain and get more space or look at other free hosting areas to see what offer