Corkyg,
In order to contribute to an IRA, you need to have earned income at least equal to the amount of your IRA contribution.

A "spousal" contribution occurs when one spouse does not have enough earned income to qualify for an IRA contribution, and still makes an IRA contribution by counting the spouse's earned income.

So for example, if spouse A makes $50,000, and spouse B makes $1,000, then the maximum IRA contribution for A would be $5,000 (since spouse A has at least $5,000 of earned income), but would be only $1,000 for spouse B (since spouse B has only $1,000 of earned income and cannot contribute more than that). However, spouse B can still make an IRA contribution under the spousal IRA rules, by counting $4,000 of spouse A's earned income to qualify. You can follow this rule as long as the total IRA contributions don't exceed the total earned income of both spouses (e.g., if spouse A earned $8,000 and spouse B earned $1,000, the maximum total contribution between them even with spousal IRAs is still only $9,000, their total income).

I hope that helps a little!