The Sanskrit word "samskara" can be translated just the way it sounds in English: as'some scars." Samskaras are energy patterns in our consciousness. I always picture them as mental grooves, like the rivulets in sand that let water run in certain patterns. Samskaras create our mental, emotional and physical default settings. The tendency to think, 'I can't do this,' when you're faced with a new challenge is a samskara, and so is the confidence that develops once you've mastered something that was hard for you. The tension lump that shows up in your right shoulder when you feel stressed is a samskara, and so are the song lyrics that pop into your mind unexpectedly and that--in my case at least-often reveal themselves to be the perfect comment on the situation you're in at the time.
Neurophysiologists mapping neural pathways in the brain report that each time we react in a certain way--falling into an anger pattern, for instance, or putting off completing a report yet one more time-we strengthen the power of that pathway. The yogic texts make the same point. The bottom line in both cases, is that the way we feel, the way we react, and the behavior we manifest at any given moment is the result of the the neural connections that are operating under the surface.
Once those neural pathways (samskaras) have been set, most people keep running down them, like rats in a maze, reacting with the same old patterns and feelings every time they find themselves in a situation that seems to mirror whatever the original trigger might have been.
You probably know, intellectually at least, how this works. When you feel abandoned because your friend hasn't called you in two weeks, you might understand that it isn't because he's stopped liking you. You may even realize, especially if you've done some therapy, that his silence is triggering one of your old samskaric grooves -- perhaps a childhood memory of abandonment. But that doesn't necessarily stop you from reacting. Samskaras are powerful, which is why knowing better doesn't always change our behavior. There's a weight to those accumulated impressions. They are, on a daily basis, the reason why we think and feel the way we do.
That's both good news and bad news. The bad news about samskaric grooves is that as long as the negative ones are in place, it's hard to escape the limitations imposed by our personal history. The good news, however, is that we can change those grooves. The brain is so fluid and malleable, so prone to take and hold impressions, that when we keep leading it into new pathways, the accumulation of new insights, practices, and experiences will eventually overwhelm the old ones, and even, given the right circumstances, eliminate them entirely.