Tags are keywords you can set on your photos. They define the picture you share. For example, you can set the people name on the picture, the location, the event, the objects. When you'll create albums, you'll understand why it's important to do it right. Be careful to stay with a few tags, don't add too much tags.
PicsEngine has the great feature to tag photos directly in the web-interface. First, select some photos (shift/alt are useful to select multiples photos). The right panel allow you to see the selected photos tags. If all the photos contain the same tag, this tag is emphasized. If some don't contain the tag, this one will be more transparent. You can set tags by clicking on them. If you want to add a tag that doesn't exists yet, insert the name in the search input and press Enter.
Because PicsEngine retrieve IPTC tags of your photos, vous can tag your photos with others softwares and then import the photos into PicsEngine.
A lot of photograph and graphic softwares allow you to do things like that like Adobe Lightroom, Picasa, iPhoto, Aperture, Adobe Photoshop.
Tags cannot have characters with accents, IPTC doesn't support this feature, PicsEngine will convert tags in non-accentuated tags.