When selecting plants for a garden, different areas will get different amounts of light/shade. A plant will have certain ideal sunlight requirements, so be best suited to the light in a certain part of your garden in addition to other things like moisture, soil, etc.
The ideal light for a plant is often listed in groups such as full sun, light shade, partial shade, dappled shade and heavy/full shade. Often a plant label/seed packet will indicate which of these groups it prefers, where most plants will prefer a decent amount of light but you can find some suitable to areas with a lot of shade.
The varying levels of shade generally mean that something is blocking the sun from reaching a plant for a certain amount of the day, where the group is based around how many hours of sun a plant would get each day. Dappled shade is the exception, where it refers to the type of shade you might get below a tree.
What is full sun?
Full sun is when an area in your garden gets a lot of sun during the day, over around 6 hours of sun in the summer. In the UK, a south facing garden with the house on the north-most part would get the most sun/least shade, so full sun plants may be more suitable than a north facing garden.
When choosing plants for full sun, you may also see these listed as suited for south facing gardens or just described as south facing garden plants.
What is light shade and partial shade?
Light shade is where there's some shade, where it would get slightly below 6 hours of sun in the summer. You might get this next to walls/trees/tall plants where there's nothing directly above a plant but it gets some shade from the things around it.
There's some cross-over here with partial shade, but generally light shade would get a decent amount of sun and at times of the day when it's brightest. Partial shade might get sun earlier in the morning/later in the day and generally get less of it, around 3 to 6 hours of sun in the summer.
What is dappled shade?
Dappled shade is where the branches/leaves of a tree block light but some still gets through. You can get plants that are suited for dappled shade that you might find in forests, or you might try and work out how many hours of sun the area below would get each day based on how much sun gets through the tree branches. The sun on the ground may be similar to partial or heavy/full shade, so you could choose plants suitable.
Also keep in mind shade caused by other things growing in your garden like trees can also affect soil moisture, where a tree's roots can cause the nearby soil to be quite dry. So, under trees for example you might use plants suited to dry shade.
What is heavy/full shade?
Heavy/full shade is where there's less than around 3 hours of sun in the summer. You might get this if a plant is near a building or something is above it, where the plant would only get light from certain angles.
When choosing plants for heavy/full shade in the UK, a north facing garden with the house on the south-most side would get the least sun/most shade, so you may see heavy/full shade plants listed as suited for north facing gardens. Most plants do require some sunlight so heavy/full shade does still mean a plant gets some sun light.