A resume serves as your introduction to a potential employer. It can answer questions and show off your qualifications -- but it can also raise questions that you need to address.
One major concern is if you have a gap in employment. That's a potential resume red flag that you should be upfront in addressing. An employment gap probably won't keep you from getting hired, but not explaining it could scare off a potential employer.
How should you address it?
If your gap is current (i.e. you have recently been out of work for more than a few months), you should address it in your cover letter. Exactly how you do that depends upon the length of time you've been unemployed and the reason for it.
For innocuous reasons like deciding to take a break after leaving a previous job, be honest about it, but don't belabor it. "After six successful years at my previous employer, I decided to make a change, and before looking for my next challenge I spent two months traveling."
If your break happened due to family reasons -- maybe you had to care for a sick relative, or you had an illness yourself -- explain it (vaguely if you want) but also present a resolution. "I have been off the job market caring for my mother, who sadly passed away last month."
That might mean sharing details an employer can't ask about in an interview, but it's important to not only explain what happened, but why it won't be an ongoing issue. An employer is not supposed to discriminate based on past illnesses, but it can happen unconsciously.
You want to make it clear that whatever the reason for keeping you out of work, it has now been resolved. Of course, nobody knows what the future brings, but being honest and open in the moment should alleviate any fears the employer has.
Of course, sometimes an employment gap exists for reasons you don't want to share. However, even if you got fired -- or spent time in prison -- it's still best to be honest, because in most cases the truth will come out through background or reference checks.
If the gap is older and you have work experience after it, you probably don't need to address it in the cover letter. You should, however, be prepared to talk about it during an interview (especially if you had a break of longer than a few months).
If you stepped away for a logical reason, own it and be upfront about how the situation resolved itself. If you took time off to goof off, try to present it as an opportunity to recharge your batteries in order to be a better employee when you reenter the workforce.
A gap in employment doesn't have to be a deal-breaker, but leaving it out there might scare off your possible employer. Use your cover letter to briefly address the situation, and be prepared to talk about it during an interview.
Don't apologize. Explain what forced the break in your work history and how you dealt with it. If you took time off to tend to an ill parent, spouse, or child, the potential employer may even consider that a positive mark in your favor.