To critically appraise is to essentially dissect published research, usually a journal article, for its validity eg is it biased, and relevance. To aid with this it is helpful to use a checklist or a scale.
There are a number of resources to help you critically evaluate a paper or article or research.
One of the best:
Trisha Greenhalgh has written the very useful How to read a paper. Contents include: Assessing methodological quality; Statistics for the non-statistician; Papers that report trials of drugs, complex interventions, diagnostic or screening tests, summarise other papers (systematic reviews and meta-analyses), tell you what to do (guidelines), tell you what things cost (economic analyses) and more. Note there are earlier editions including an ebook of the 5th ed 2019.
'This playlist includes seven modules that address critical appraisal concepts and methods for six different research designs. In each video, we walk through the CASP checklist alongside an open access research article, to demonstrate how to approach critical appraisal in practice.'
Using a guideline or framework can help researchers with the quality of research and keep the process and methods transparent. The EQUATOR network website has a list of guidelines for main study types eg randomised controlled trials, observational studies, qualitative research, prediction models and more. See also ENTREQ for qualitative research.