what is grid & flexbox in css code example

Example: flexbox grid

.container {
    max-width: 1335px;
    margin: 0 auto;
}
.grid-row {
    display: flex;
    flex-flow: row wrap;
    justify-content: flex-start;
}
.grid-item {
    height: 550px; /*optional*/
    flex-basis: 20%; /* initial percantage */
    -ms-flex: auto;
    width: 259px; /*optional*/
    position: relative;
    padding: 10px; /*optional*/
    box-sizing: border-box;
}
@media(max-width: 1333px) {/*xl*/
	.grid-item {
    	flex-basis: 33.33%;
    }
}
@media(max-width: 1073px) {/*lg*/
    .grid-item {
    	flex-basis: 33.33%;
    }
}
@media(max-width: 815px) {/*md*/
	.grid-item {
    	flex-basis: 50%;
    }
}
@media(max-width: 555px) {/*sm*/
	.grid-item {
    	flex-basis: 100%;
    }
}