Simply put: Lower frequencies (700Mhz and below) can penetrate walls and cover a wider area while higher frequencies (above 700MHz) are faster at transmitting but have shorter reach.
This illustrates how a lower frequency can penetrate through walls:
But if you want a good grasp of the idea and go geeky, we need to science it up a bit.
The radio spectrum is the invisible highway of airwaves that traffic TV, radio, smartphone or wireless capable device. Kinda like an EDSA for your texts, calls, TV, etc.
In this highway, there are "lanes" for each kind of user.
The lower spectrum (500-700 MHz) is used for broadcasting TV and Radio. Good reach but are slow in transmitting. Imagine this as a "lane" exclusive for light vehicles that can travel far like motorcycles and small cars.
Mobile Networks are at the mid spectrum (700 - 2600 Mhz). They use it to service your calls, texts, and mobile internet. Think of this "lane" being exclusive to mid-size vehicles that can carry more stuff like SUVs and sedans.
While the higher spectrum (2.4 - 5 Ghz) is used for high speed wireless internet. This "lane" is reserved for the heavy carrier types like trucks and trailers.
Transforming the 700 Mhz from broadcast to mobile telco means adding extra wide and reliable "lanes" to the ones being used by Smart.
This means more "lanes" dedicated to better reach and signal for day-to-day calls, text messaging, and mobile surfing.
These "extra lanes" will also bring stronger signal to cities, towns, and beyond.
Thanks to these "lanes" indoor users at home and in offices can also expect better coverage and less drops.
Now, the next time somebody asks you about this 700 Mhz biz, school these fools with science!