The tags in question are META tags. They are the bits of code that sit in between the <HEAD> </HEAD> tags and tell search engines whats in a page (except google which actually reads a pages text content).
Meta data should be put just after the <TITLE> tags in the <HEAD>. It tales the format <META name="the catagory" content="the information">. The name is usually one of 4 types - "description", "resource-type", "keywords" or "distribution". The information depends on the name. I will briefly cover the two that are most important for a search engine: "description" and "keywords". The "keywords" are a list of main topics covered by the page in question. This blog has the keywords "photoshop, illustrator, dreamweaver, Hints, Tips, Techniques, Carl Topham" since thats the main list that covers it. Separate each keyword with a comma (and space). Keywords can be two words so "Carl Topham" is counted as one keyword. Try to keep the keywords restricted to less than about 250 characters as some search engines actually reduce page ranking if you use too many. The "description" is just that. It's a more people friendly description of your page. Try to sumarise in 2-3 sentences. Some search engines use it but its usually just the bit shown under your link in the search engine. You should end up with code that looks like:
<META NAME="Keywords" CONTENT="illustration, Carl Topham, photoshop, illustrator, dreamweaver, Hints, Tips, Techniques"> <META NAME="Description" CONTENT="The official blog of Carl Topham. Containing a range of graphic design, web design....">
Save and upload to your server. If you are using wordpress the you need to login and goto "presentation > theme editor" and choose "header" on the page list on the right. Then paste it on a new line after the <TITLE> tags same as in any other editor. "Update file" and your done. Now wondering search engine spiders and robots should add you with the relevent tags.