I read in a book- "a fluid is a system of particles loosely held together by their own cohesive forces or by the restraining forces exerted by the walls of a container"... I cannot understand the latter part (restraining...........container), Please help?
Well, fluids flow by definition........
We can cup our hands together, and drink water from our hands; our hands form a (leaky!) container. Of course, it is better, (and more polite!), to take a mug or a glass, and fill this with water, or tea, or coffee, or wine, or whatever your poison, and sip from the container with pinkies outstretched. The liquid is a condensed phase with definite volume.
On the other hand, gases, while still fluids, have NO fixed volume, and will expand to fill uniformly the volume of whatever container you put them in. As you know, (and might have done yourself), scuba divers fill their tanks with air from a compressor. And they need a very sturdy tank to contain the elevated gas pressure.
If I missed the point of your question, come back and someone will help you.