If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian,
... the total luminous flux emitted into that angle is one lumen.
Alternatively, an isotropic one-candela light-source emits a total luminous flux of exactly 4π lumens.
If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens.
The luminous intensity would still be one candela in those directions that are not obscured.
The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source.
The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.
The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread.
A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux.
The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux.
Mathematically, 1 lx = 1 lm/m2.
A single fluorescent light fixture that produces a luminous flux of 12000 lumens might light a residential kitchen with an illuminance of 500 lux.
Lighting a larger area to the same illuminance requires a proportionately greater number of lumens.
Have a good weekend!