Personas are characters created to represent the target group. Their personnality, occupations, motivations...(what they think, do, feel) can be described. Research is needed to get the necessary data to create them. They should be the "representatives", the typical users of the different groups of people targeted by a product, service, website...
When creating Personas, it is important to not limit yourself to the "mainstream" users. You should also think about some more "extreme" ones. Depending on the project, 3-5 personas can be enough. Don't use "boring" stock photos or a simple bullet list to describe them. It is better to write a nice narrative that will humanize them: they need to seem real, alive and identifiable. It is not about making personas as age groups, genders or location but more about behaviors: you can use in the same way trains, whether you are 25 or 46, a woman or a man... So focus on behaviors! To make it more visual, you should use 5-8 "behavioural sliders": this kind of sliders with scales and opposite data on each side like "Using trains per month: 0--5--10--20--30--More" or with emotional values.
You could also list some problems each persona has to face, and write down a few solutions you can already think about (but don't focus much on these solution during the rest of the process!)
Personas help the team to be on the same line, have a good understanding of the target group. Designers can refer to them during the process to create user stories, evalute ideas, during co-creative sessions and any time a team members or client need to remind the target.