A travel insurance is an insurance that is intended to cover financial and other losses incurred while travelling, be in within one's country, or internationally.
Travel insurances can usually be arranged at the time of booking of a trip to cover exactly the duration of that trip or a more extensive, continuous insurance can be purchased from (most often) insurance companies or banks, which covers all travelling during the period for which the insurance was obtained.
Baggage is the reason most people take out travel insurance and is actually probably the least important component; things can be replaced, health can't. There are usually per item limits and exclusions regarding the definition of 'reasonable care' which means a claim will be denied if you leave your baggage unattended in a public place and it's stolen. Lastly, should you have to claim, you will usually be asked for proof of ownership in some form, so you might like to consider this before you go.