What is it called when you use many words to describe the word you want to use?

If you don’t know the word ‘bungalow’, and you say ‘It’s a kind of house’ or ‘It’s a sort of house’ then this is approximation; an approximation is anything that is similar but not exactly equal to something else. The term can be applied to various properties (e.g. value, quantity, image, description) that are nearly but not exactly correct; similar, but not exactly the same (Wikipedia).

You are looking for circumlocution.

Circumlocution (also called circumduction, circumvolution, periphrasis, or ambage) is locution that circles around a specific idea with multiple words rather than directly evoking it with fewer and apter words.

• For example, governmental income support to poor residents might be referred to as "welfare".

• Roundabout speech refers to using many words (such as "a tool used for cutting things such as paper and hair") to describe something for which a concise (and commonly known) expression exists ("scissors").

• The vast majority of definitions found in dictionaries are circumlocutory.

[Wikipedia]

OED definition of "circumlocution: "The use of many words where fewer would do, especially in a deliberate attempt to be vague or evasive".

Rambling on may be a valid answer:

to talk or write about something for a long time in a way that is boring, annoying, or confusing

If none of the previous answers were quite what you were looking for, you might have been thinking of pleonasm:

pleonasm [plee-uh-naz-uhm]

noun

1. the use of more words than are necessary to express an idea; redundancy.
2. an instance of this, as free gift or true fact.
3. a redundant word or expression.

(from Dictionary.com)

Some applicable synonyms that have not yet been mentioned include: garrulity, logorrhea (which is just a really funny word), loquaciousness, loquacity, prolixity, verbiage, verboseness, verbosity.