You all go for dinner (say 3 people). Everyone orders food and start eating...
Person 1 - Takes 45 mins for eating
Person 2 - Takes 20 mins for eating
Person 3 - Takes 55 mins for eating.
Everyone can leave only after person 3 completes his/her food.
So the longest path would be reaching restaurant --> Ordering food --> Person 3 finishing the dinner --> paying bills
This is also the shortest time they can complete the dining.
Now lets explain float as well
Person 3 cannot have any slack. Meaning - he cannot attend any phone calls/use restroom, if he/she does take a break it will still delay the dining.
Whereas person 2 and person 1 can take a break/do other things while they are eating.
So activities in critical path ideally cannot have float.
Training for Project Management Professional (PMP)®, PMI Agile Certified Practitioner (PMI-ACP)®, and Certified Associate in Project Management (CAPM)®