Touch target is the area in which the user can effectively tap to invoke an action. If the size of the touch target is not sufficient different user groups may find it difficult to tap on a specific element of the screen. A low vision user may not be able to point their finger on a specific element, users who have trimmers in the hand may accidently touch the neighboring element or a user who have fat fingers may not be able to use it effectively.
WCAG 2.1, SC 2.5.5 Target Size (Level AAA) talks about this requirement. As per this success criteria, the user interface element should have a touch target size of 44 by 44 CSS pixels. The size can either achieved either by the size of the UI element or along with the element and the padding around it which may also be an untappable area.
Exceptions
If the action performed by the UI element is available in other means on the same screen where the touch target size is 44 by 44 pixels;
If the target element is part of a sentence or block of text;
If the element is controlled by the user agent and the content author has no control over the element’s target size;
If the particular presentation of the target is essential to the information being conveyed;
Additional requirements by the OS
Android
The Android Material Design Accessibility guidelines expects at least 48 by 48 DP of the touch target. Any on-screen element that someone can click, touch, or otherwise interact with should be large enough for reliable interaction. Consider making sure these elements have a width and height of at least 48dp. Additional information can be found here.
IOS
The IOS design guidelines needs at least 44 by 44 pt target size for the UI elements on the screen.
WCAG Success Criteria
2.5.5 Target Size (Level AAA) WCAG 2.1