Scanning a line of poetry or an entire poem means that you are being asked to mark out the rhythm and meter of the poem. Any line of poetry is going to have words that contain stressed and/or unstressed syllables. Many poets will try to pick a syllable pattern and repeat it throughout each line in the entire poem. The repetition of that syllable pattern creates rhythm. Each unit of rhythm is called a "foot."
The question states that "To My Dear and Loving Husband" is written in iambic pentameter. That is correct, although line 10 does slightly deviate. That means each line of the poem contains the iambic foot. An iambic foot contains an unstressed syllable followed by a stressed syllable. Some common iambic words are the following: behold, awake, destroy, and employ. Each line of "To My Dear and Loving Husband" is a repetition of the unstressed/stressed syllable pattern. That is the poem's meter. Each line of the poem contains five iambic feet, which is why it is called iambic pentameter.
Typically when a person scans a line of poetry, he/she marks out each stressed and unstressed syllable with a mark above the syllable. A "-" means unstressed and a "/" means stressed. The line that the question asks about would look like the following:
- / - / - / - / - / I prize thy love more than whole mines of gold,
If that's not clear, it's possible to scan a line of poetry by using capital letters for the stressed syllables. For example,
i PRIZE thy LOVE more THAN whole MINES of GOLD,
This method is more awkward to type and read, so I don't prefer it.
You could also use bold to mark out the stressed syllables:
I prize thy love more than whole mines of gold