Welcome to the forums!
you have certainly come to the right place as you will find all you need to know on budgie care here.
Since you seem relatively new to budgies, the information on the several articles and sticky threads at the top of each section of the forums will be extremely useful as you become more acquainted with the species.
Now onto your questions, if your budgie willingly steps onto your finger when offered, chooses to be close to you and sings in happiness, preens your fingers/hand or face, all of this means your budgie likes you. A budgie's tail feathers can momentarily fan out of excitement, especially when accompanied by some enthusiastic singing and head bobbing.
"Non rigid" means that they aren't scared stiff nor do they look like statues when very stressed and scared.
Their bodies and overall stance are relaxed and their body movements flow natural and effortlessly. This is something easy to spot and decipher by spending time with a pet bird.