An alkene is a hydrocarbon containing at least one carbon-carbon double bond, and that double bond holds a genuinely exposed, electron-rich region sitting above and below the plane of the two bonded carbons, a region other molecules can attack far more readily than they can attack an ordinary single bond. Addition reactions exploit exactly that reactivity: rather than substituting one atom for another, an addition reaction breaks the double bond open entirely and adds a new atom or group to each of the two carbons, converting the alkene into a more stable, fully single-bonded product.
The double bond's exposed electrons make it a target rather than a stable resting point
A carbon-carbon double bond is actually made of two different types of bond stacked together, a stronger bond running directly between the two nuclei and a weaker, more exposed second bond formed from electron density sitting above and below that line. That weaker component is considerably easier to break than an ordinary single bond, and it leaves a genuinely accessible cloud of electron density sitting outside the direct line between the two nuclei, exactly where an approaching molecule looking for electrons to bond with can most easily reach it.
Breaking the weaker bond lets each carbon pick up an extra atom or group
When an addition reaction occurs, that exposed, weaker part of the double bond breaks first, and the electrons it releases form two brand new single bonds, one linking each of the original two carbons to an atom or group from whatever reagent has attacked the molecule, hydrogen, a halogen, water. The double bond disappears entirely in the process, replaced by a single bond between the two original carbons plus two new bonds outward, and the resulting fully single-bonded product is generally more chemically stable than the alkene it started as, which is exactly why addition reactions tend to proceed readily once the right reagent is introduced.
What we're still unsure about
That an alkene's double bond is genuinely more reactive than a single bond, and that addition reactions exploit that reactivity to convert an alkene into a more stable single-bonded product, are well established, thoroughly confirmed principles of organic chemistry. What's more genuinely a matter of ongoing mechanistic detail is exactly which of two possible carbons a new atom or group ends up attaching to when the alkene isn't perfectly symmetrical, since the reaction can proceed through more than one possible intermediate step, and chemists continue to refine predictive rules, and continue finding real, instructive exceptions to those rules, for exactly which product actually dominates in a given reaction, rather than a single simple rule correctly predicting every case.
This sits inside Addition Reactions & Alkenes, one of eight topics in Organic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.