Sensory means the things that we senses and the processing skills we use to interpret those things...sight and visual processing, hearing and auditory processing, touch (really, this effects all processing, but for the purposes of your question, most often it is how children react to certain clothing items, air temperature, and textures) balance and body awareness (vestibular processing) and a motor component to each of the different kinds of processing along with interaction with all the intlectual processing. They are all intertwined in ways you have never really thought about, until one of these is "off."
Many kids with neurodevelopmental issues will also have senosory processing diffiuclties of one kind or another. It is common for sensory issues to be one presentation of a child's profile, but some professionals now think that sensory processing disorder also stands alone. As an educational advocate, I have seen many kids with sensory concerns, and most are part of and secondary to a primary diagnosis like autistic spectrum, ADHD, tourette, and others.
Kids may out grow or learn to tolerate things that they find overstimulating or unbearable as children. Some persist with the same aversions or difficulties through life. Visual processing issues can be very debilitating for a lifetime if not remediated well. Outcomes are very differnet for every child and "sensory" covers a very broad range of presentations.
You probably knew people with issues, but did not know it. Kids with visual processing issues may be clumsey, or not read well or not be able to write neatly. Kids with vestibular processin issues may always stand too close, or bump into others and wil avoid some activities or seek out swinging, or hanging...did you ever see a kid roll around on the floor when they were supposed to be listening to the music or the book at circle time? Have you ever seen a kid with their shoes on the wrong feet, becaue they like it that way, or not wearing socks in the winter, or not wearing his coat and hat, but dragging them through the snow while his Mom yells at him to put it on? Those were sensory issues, more than likely.
They were there when you were younger too.
M.