what is flexbox used for code example

Example 1: what is html used for

<!-- Answer to: "what is html used for" -->

<!--
  HTML is used to create electronic documents (called "pages")
  that are displayed on the World Wide Web.
-->

Example 2: flexbox css

.container {
  align-content: flex-start | flex-end | center | space-between | space-around | space-evenly | stretch | start | end | baseline | first baseline | last baseline + ... safe | unsafe;
}

Example 3: flexbox

Display: flex 
Flex-direction: row | row-reverse | column | column-reverse
align-self: flex-start | flex-end | center | baseline | stretch
justify-content: start |  center | space-between | space-around | space-evenly

Example 4: flexbox css

.container {
  flex-flow: column wrap;
}

Example 5: why %u is used for ?

%d is used to specify an integer, %u is used to specify an unsigned integer .

Example 6: flexbox

Please read flexbox & grid in CSS TRIX, 
make hands-on project watching (WESBOS videos - it's really useful)
also try [FLEXBOX - froggy] and [GRID - garden] games