Note that if you want to have stylized text (bold, different fonts, etc) in the popup window you can use HTML. Here's an example:

import folium
import pandas

data = pandas.read_csv("Volcanoes.txt")
lat = list(data["LAT"])
lon = list(data["LON"])
elev = list(data["ELEV"])

html = """<h4>Volcano information:</h4>
Height: %s m
"""

map = folium.Map(location=[38.58, -99.09], zoom_start=5, tiles="Mapbox Bright")
fg = folium.FeatureGroup(name = "My Map")

for lt, ln, el in zip(lat, lon, elev):
    iframe = folium.IFrame(html=html % str(el), width=200, height=100)
    fg.add_child(folium.Marker(location=[lt, ln], popup=folium.Popup(iframe), icon = folium.Icon(color = "green")))


map.add_child(fg)
map.save("Map_html_popup_simple.html")

You can even put links in the popup window. For example, the code below will produce a popup window with the name of the volcano as a link which does a Google search for that particular volcano when clicked:

import folium
import pandas

data = pandas.read_csv("Volcanoes.txt")
lat = list(data["LAT"])
lon = list(data["LON"])
elev = list(data["ELEV"])
name = list(data["NAME"])

html = """
Volcano name:<br>
<a href="https://www.google.com/search?q=%%22%s%%22" target="_blank">%s</a><br>
Height: %s m
"""

map = folium.Map(location=[38.58, -99.09], zoom_start=5, tiles="Mapbox Bright")
fg = folium.FeatureGroup(name = "My Map")

for lt, ln, el, name in zip(lat, lon, elev, name):
    iframe = folium.IFrame(html=html % (name, name, el), width=200, height=100)
    fg.add_child(folium.Marker(location=[lt, ln], popup=folium.Popup(iframe), icon = folium.Icon(color = "green")))

map.add_child(fg)
map.save("Map_html_popup_advanced.html")