谷

gu

valley; grain

谷物 Grains; 五谷丰登 Bumper harvest (lit. five grains abundant)

峡谷 Canyon; 低谷 Low point (metaphorical, e.g., in life)

A split (八) in the earth with a mouth (口) as the entrance in a  valley ( 谷)

宀

mian

容

rong

to contain, to hold

to allow / tolerate

countenance , appearance

容纳 to accommodate, to hold; 容忍 to tolerate, to endure;

内容 content; 笑容 smile (facial expression)

The 宀 suggests an enclosed space, while 谷 may imply a hollow or spacious area, like a valley, where the things are contained;

俗

su

custom, common practice

vulgar, secular

风俗 traditions;  世俗 worldly, secular; 

庸俗 vulgar (moral/cultural shallowness); 俗气 vulgar (aesthetic tackiness)

a human (亻) behavior, which is widespread and low-lying (谷), thus becoming common among the masses