Investing, broadly, is putting money to work for some time in some sort of project or undertaking to generate positive returns (i.e., profits that exceed the amount of the initial investment). It is the act of allocating resources, usually capital (i.e., money), with the expectation of generating an income, profit, or gains.
One can invest in many types of endeavors (either directly or indirectly) such as using money to start a business, or in assets such as purchasing real estate in hopes of generating rental income and/or reselling it later at a higher price.
Investing differs from saving in that the money used is put to work, meaning that there is some implicit risk that the related project(s) may fail, resulting in a loss of money. Investing also differs from speculation in that with the latter, the money is not put to work per se, but is betting on the short-term price fluctuations.